1. Field of the Invention
The present invention relates to implantable medical devices that are operated by a radio frequency signal which is received by the device, and more particularly to biologically compatible antennas for such devices.
2. Description of the Related Art
A remedy for people with slowed or disrupted natural heart activity is to implant a cardiac pacing device which is a small electronic apparatus that stimulates the heart to beat at regular rates.
Typically the pacing device is implanted in the patient's chest and has sensor electrodes that detect electrical impulses associated with in the heart contractions. These sensed impulses are analyzed to determine when abnormal cardiac activity occurs, in which event a pulse generator is triggered to produce electrical pulses. Wires carry these pulses to electrodes placed adjacent specific cardiac muscles, which when electrically stimulated contract the heart chambers. It is important that the electrodes be properly located to produce contraction of the heart chambers.
Modern cardiac pacing devices vary the stimulation to adapt the heart rate to the patient's level of activity, thereby mimicking the heart's natural activity. The pulse generator modifies that rate by tracking the activity of the sinus node of the heart or by responding to other sensor signals that indicate body motion or respiration rate.
U.S. Pat. No. 6,445,953 describes a cardiac pacemaker that has a pacing device, which can be located outside the patient, to detect abnormal electrical cardiac activity. In that event, the pacing device emits a radio frequency signal, that is received by a stimulator implanted in a vein or artery of the patient's heart. Specifically, the radio frequency signal induces a voltage pulse in an antenna on the stimulator and that pulse is applied across a pair of electrodes, thereby stimulating adjacent muscles and contracting the heart.
The stimulator in that wireless system is powered by the energy of the received signal thus requiring that the pacing device transmit a relatively strong radio frequency signal in order to provide adequate energy to the stimulator implanted deep in the patient's chest. It is desirable to place the stimulator, or at least the antenna for the stimulator, in a blood vessel located closer to the skin of the patient with electrodes implanted in one or more cardiac blood vessels and connected to the stimulator by wires extending through the electronic circuit circulatory system. This would enable more of the energy from the frequency signal to reach the stimulator, however, the blood vessels close to the skin are not sufficiently large to accommodate the size of the stimulator.
The antenna, usually in the form of a coil. must possess several characteristics in order to function within the blood vessel. The coil must retain its shape in order to remain tuned to the particular radio frequency being used. The conductors of the antenna have to be insulated so that the blood and other substances flowing through the vascular system do not provide a short circuit or otherwise detune the antenna. In addition, the antenna must be biologically compatible with the blood vessel walls and with the blood.